The invention relates to cellulose derivatives.
The term "cellulose derivatives" is understood here to refer to the compounds formed, after chemical reactions, by substitution of the hydroxyl groups of cellulose, these derivatives also being referred to as substitution derivatives. The invention relates in particular to cellulose formates and to solutions of cellulose formates.
Processes for obtaining cellulose derivatives and/or solutions of these derivatives, as well as certain specific steps of these processes, have been described in a large number of documents. Reference will be made, in particular, to patent EP-B-179,822. That document describes the production of anisotropic spinning compositions based on cellulose formate, by reaction of cellulose with formic acid and phosphoric acid, as well as of the fibers with high-quality mechanical properties obtained from these compositions, it being possible for these fibers to be regenerated.
More specifically, the invention relates to processes for manufacturing cellulose formates and solutions thereof, in particular solutions which can be spun, when these processes are carried out using a cellulose pulp industrially conditioned in the form of plates.
These plates are, in a known manner, rigid sheets whose thickness is generally between 0.2 mm and 5.0 mm, usually about 0.5 to 2 mm, in which the cellulose is in fibrous, relatively compacted form depending on the desired density for these plates. These plates are generally in a flat, rectangular, cut form or in a form rolled up on themselves in order to form continuous (narrow) strips.
A distinction is usually made, by convention, between so-called "low-density" plates, whose density is less than 0.5 g/cm.sup.3, and so-called "high-density" plates, whose density is at least equal to 0.5 g/cm.sup.3.
In the industry of cellulose transformation, and in particular in the industry of cellulose fibers and films, these are plates which are easy to transport and store, and are used essentially as base materials. Such plates or their use as starting material have been described, for example, in the following patents or patent applications: EP-A-251,674, FR-A-2,678,625, U.S. Pat. Nos. 2,105,498, 2,393,783, 2,644,818, 4,211,574, 4,336,370, 4,343,840, 4,840,673, 5,036,900, 5,114,535.
When such cellulose plates are used as starting material, all the known methods for obtaining cellulose derivatives and/or solutions thereof, as varied as they are, require the same prior step of mechanical destruction of these plates, this operation being intended to make them lose their unity and cohesion, by separating the fibers constituting them in order thereby to make the cellulose accessible to the various reactants used.
The term mechanical destruction is understood to refer very generally here to any operation of disintegration in general, whether it is total disintegration, that is to say reduction to powder obtained by a pulverization technique, or partial disintegration obtained by shredding, partial grinding, attrition or any equivalent action, such disintegration operations being carried out using known suitable tools or machines such as, for example, mincers, milling machines, grinders or shredders.
The reduction of cellulose plates to powder is a common operation. It is generally carried out independently of any other process, it being possible for the powder thus obtained to be stored before being processed. The powder moreover has the advantage of good chemical reactivity and of being commercially available. The use of powder is thus totally widespread, in particular for laboratory-scale tests which require only limited amounts of material.
Besides the pulverization costs themselves, this reduction to powder has many other drawbacks. One major drawback lies in the safety problems associated with the explosion or fire risks. These risks, which are inherent in operations for pulverization, handling or storage of powders in an uncontrolled atmosphere, have been mentioned, for example, in U.S. Pat. No. 5,036,900. They require the use of expensive safety and control devices. Other known drawbacks are associated with the loss of material in the form of dusts, with the very presence of these dusts and with the transportation and storage problems posed in the face of a considerable increase in the volume of the starting material after it has been pulverized. All of these constraints are difficult to accommodate, in economic terms, for large-scale industrial manufacture.
The partial disintegration methods, as mentioned above, have, for the various reasons outlined above, been favored in a large number of processes. Only a few examples will be mentioned.
Document FR-A-2,678,625 describes a process for the production of cellulose acetate, in which a high-density cellulose plate is disintegrated using various grinders. U.S. Pat. No. 2,393,783 describes the destruction of plates and the dispersion of the individual fibers by subjecting these plates to a violent blow of compressed air, before esterification. U.S. Pat. No. 2,644,818 describes a process for producing cellulose ethers with a specific step of shredding alkalicellulose plates. U.S. Pat. No. 5,036,900 describes a machine designed to shred high-density cellulose plates, before a step of acetylation.
The industrial processes most widely known and developed such as, for example, those known as the acetate process or the xanthate or viscose process, from the name of the cellulose derivatives obtained or from the name of their solutions, also make use of these techniques of partial disintegration of plates. This disintegration is thus a fully integral, entirely separate step of these processes.
However, all of these partial disintegration methods themselves have a certain number of drawbacks. Besides the actual machine costs, a major drawback lies in the risk of deactivation (loss of chemical reactivity) of the cellulose during disintegration. In order to avoid or limit this deactivation, it will be necessary, for example, to disintegrate the plates using specific, and restricting, conditions of drying and wetting the material. In certain cases, a reactivation treatment of the cellulose may even be necessary. Other drawbacks lie in the risks of degradation of the cellulose (such as depolymerization, for example), and losses of material in the form of dusts. Needless to say, all of these drawbacks have a negative impact on the final industrial cost. Some of them have been mentioned in particular in the following documents: FR-A-2,678,625, U.S. Pat. Nos. 2,105,498, 2,393,783, 5,036,900, 5,114,535.